Those of us with pets such as dogs know that one of their favorite places to rest or sleep is upon a floor mounted HVAC register. Such registers put out warm air in the wintertime and cool air in the summertime which helps the animal regulate their body temperature as well. This keeps the animal comfortable beyond temperature ranges that would be acceptable to humans.
However, this pet behavior does have several disadvantages. First, the pet is forced to lie upon the floor or vent without any padding or even a blanket which can be very uncomfortable. Second, the vast majority of the air is blocked off, thus affecting temperature in the room which can cause the room occupants to become uncomfortable. Finally, should the pet be large enough or positioned just right, the entire air flow can become blocked. When this occurs, the pet receives no temperature relief as well as the room occupants.